1. Field of the Invention
The present invention relates generally to improvements in a temperature-controlled fan fluid coupling which controls the rotation of a fan that cools an automotive engine by supplying an appropriate amount of cooling airflow to the engine always according to the operating conditions.
2. Description of the Prior Art
A conventional fan fluid coupling of this kind is shown in FIG. 13, where an enclosed housing is comprised of a cover 23' and a casing 23". The inside of the housing is partitioned into an oil reservoir chamber 25 and a torque transmission chamber 26 by a partition plate 24 provided with an outflow control hole 24'. A driving disk 22 is mounted inside the torque transmission chamber 26, which is in communication with the oil reservoir chamber 25 via a dam 28 acting as a pumping portion. A circulation passage 27 extends from the dam 28 and has an entrance port 27' and an exit port 27". In order that at least one of these ports be located above the level of the oil in the oil reservoir chamber 25 irrespective of the stopped condition, the exit port 27" is formed at the open end of a substantially arc-shaped groove 29 communicating with the circulation passage 27. The groove 29 is formed by holding a partition wall around the inner wall surface of the oil reservoir chamber 25.
In this prior art fan fluid coupling, if the circulation passage 27 is submerged in the oil inside the oil reservoir chamber 25 when the vehicle is at rest, oil is prevented from spontaneously flowing back into the torque transmission chamber 26 from the reservoir chamber 25 through the passage 27 to prevent collection of oil inside the transmission chamber 26. In this way, the rotational speed of the fan is kept from increasing violently immediately after the engine is started. Therefore, abnormal fan noise is prevented. Also, during cold weather the engine is effectively warmed up. When the engine is operating at high temperatures, the outflow control hole 24' in the partition plate 24 is opened by a valve member, and this hole 24' is submerged in the oil stored in the oil reservoir chamber 25. Under this condition, if the engine is stopped, then oil spontaneously flows out of the chamber 25 through the outflow control hole 24' and a large amount of oil is collected in the torque transmission chamber 26 while the engine is at rest. Accordingly, if the engine is then restarted, the rotational speed of the driven fan increases after the lapse of a certain time as indicated by the performance characteristic curve B in FIG. 14.
In the aforementioned prior art fan fluid coupling, only the centrifugal force produced by rotation forces oil out of the torque transmission gap of the torque transmission chamber and so oil flows slowly through this gap. Heat is produced by shear for a long time, thus elevating the temperature of the oil. Also, the oil is not quickly circulated through the coupling. Since a sufficient amount of heat is not conducted to the outside, the viscosity of the oil changes, or drops. As a result, the fluid coupling fails to act adequately in response to the ambient temperature. Further, hunting takes place possibly because oil does not smoothly flows into the circulation passage due to oil pressure variations around the dam. In addition, the aforementioned increase in the rotational speed of the driven fan occurs for some time.